Interrupting ratings

[wroggent]

First, am I correct in believing that (3 pole) low voltage circuit breakers have both a 3 pole and single pole interrupting rating? If so...

Generally, are the interrupting ratings provided in technical literature for MCCBs, ICCBs, and LVPCBs and the like a 3 pole interrupting rating, and is not necessarily equal to the single pole rating? To me it seems like usually the only information provided about interrupting ratings are that they are symmetrical. Here's an example:

http://www.eaton.com/ecm/idcplg?IdcService=GET_FILE&dID=167307

(or google "eaton magnum ds" and click 4th result)

Do single pole interrupting ratings need to be considered for solidly grounded WYE systems? Said another way, do single pole interrupting ratings only need to be considered in systems where the line-ground voltage can be as high as the line-line voltage? It seems that it is only ever suggested to investigate the adequacy of the single pole rating in high resistance or corner grounded delta systems - for solidly grounded wye it seems most people simply check the 3 phase fault level and select a device with a high enough 'interrupting rating', as it is shown in the manufacturer's manuals.

Do they need to only be considered for slash rated, e.g. 480/277, devices or also for non-slash rated, e.g. 600 or 480.

Is it true to say that single pole interrupting ratings are generally not considered because it is assumed that the single line ground fault level will be sufficiently low? They are considered in 'special case' systems because a single pole may need to interrupt a line-line fault which is assumed not not be low magnitude?

NEC 110.9 applies to both single pole and 3 pole ratings, correct?

For spot networks with solidly grounded wye secondaries, should consideration be given to the single pole rating?

Unrelated to the above: Are network protectors simply breakers with a shunt trip and 32 relay or is there some other difference?

 

[dpc]

See the FPN at NEC 240.85. For solidly-grounded systems, the single pole interrupting rating is not a concern. The main problem is with corner-grounded systems, since the single pole will see a full 480 V.

 

[wroggent]

So you're in the school of thought that it's dependent on the potential L-G voltage? In tim33333's coordination thread you recognize the fact that for delta/wye transformers the LG fault can be higher than the three phase fault. For a circuit breaker mounted directly to secondary terminals of delta/wye transformer would you consider the single pole rating then?

 

[dpc]

Breakers are rated to interrupt fault current at a certain voltage. If you apply a higher voltage, the interrupting rating is generally lower.

For a solidly-grounded system, I would not worry about the single-pole rating.

 

[LionelHutz]

I've never seen a single pole rating for a circuit breaker. The interrupt rating given is for 1, 2 or 3 of the poles interrupting the fault current and the rating typically varies depending on the applied voltage.

 

[unclebob]

I guess opening a single pole fault is easier than a 3 phase fault. Let's say we have a single fault current at 40 kA and a 3-phase fault of 32 kA.
Witch one is the easier to open: 40 kA on one pole or 3 x 32 kA on 3 poles?

 

[mastergate]

French standard NF C15-100 / Section 533.3.1 requires this verification.
This standard states for IT systems (isolated neutral): Additionaly, protective devices shall be able to interrupt under single pole, under composed voltage Ph-Ph, double fault current.>
Few manufacturers give information about single pole interrupting rating, however, the french standard gives some assumptions.
Calculation software ALPI CanecoBT, for example, takes into account this verification.

For the short circuit condition, IEC 60364-5-53 (part 534.2.3.5) considers the combination of surge protective device SDP and overcurrent protective device:
The short-circuit withstand of the SPDs (in case of SPD failure) together with the specified associated (internal or external) overcurrent protective device shall be equal to or higher than the maximum short-circuit current expected at the point of installation taking into account the maximum overcurrent protective devices specified by the SPD manufacturer.
In addition, when a follow current interrupting rating is declared by the manufacturer, it shall be equal to or higher than the expected short-circuit current at the point of installation.
SPDs connected between the neutral conductor and the PE in TT- or TN-systems, which allow a power frequency follow-up current after operation (e.g. spark gaps) shall have a follow current interrupting rating greater or equal to 100 A.
In IT systems, the follow current interrupting rating for SPDs connected between the neutral conductor and the PE shall be the same as for SPDs connected between phase and neutral.>